Currently the apps that I've found the most benefit from are:
Juice Defender - Battery Power Manager
Juice Plotter - Battery Power Monitor
Lookout - Security App
IHeartRadio - Internet Radio App
KeyRing - Retail Store Keychain (Barcode Scanner for Member cards)
I'm sure that there are a ton more VERY useful apps. Please share any that you have found to be most beneficial to your SmartPhone usage.